1. Field of the Invention
This invention relates to improvements in traction drives and more particularly to providing a drive with improved volumetric, envelope and weight characteristics as well as providing a convenient means of changing the turns ratio.
2. Description of the Prior Art
Traction drives utilize smooth continuous surface rolling elements to effect a given turns ratio between an input and an output member. The drive components are usually lubricated with a traction fluid such as Monsantos Santotrac fluids. These are typically synthetic high traction cycloaliphatic hydrocarbons. The traction coefficient of the fluid is approximately 50 percent greater than available from conventional mineral oils. This coefficient represents the maximum available traction forces which would be available at the interfaces of the contacting components and is a measure of the maximum available drive torque. When this torque is exceeded, unacceptable gross slippage occurs. A slip of less than 3 percent is normal for most traction drives. Drives are sometimes advertised as "non-slip" but in reality always have some small value of micro-creep.
The various drive components in conventional traction drives are shaped in a variety of ways. Most common are rings, cones and cylinders. Many of these drives are combined with conventional gear components to effect a given turns ratio.
Conventional traction drives present a number of undesirable characteristics. For instance, many are bulky and present undesirable diameter, length and volume envelopes such as is the case with a nutating drive assembly which uses double cones rolling and nutating inside two fixed rings. The drive components of this concept are also complex to fabricate. Another bulky, complex transmission, uses four fixed cones which are loaded against, and impart rotation to a movable roller. This concept also uses a combination of gearing and traction drive to effect a total turns ratio.
Another example is a twin pulley transmission which uses a pair of steel V-belts to tractively drive an output shaft. This drive is also bulky and complex even when limited to a single drive ratio.
The NASA Lewis Research Center has developed a traction drive which consists of a single stage planetary roller with two rows of five stepped planetary rollers contained between concentric sun and ring rollers. This transmission is compact but does not offer the advantages of turns ratio adjustment.